The good news is that most cracks caught early (under 1/16" wide and less than 3" long) can be repaired in a single afternoon for under $50. The bad news is that a crack ignored for two winters may require removing the window, replacing rotted framing, and re-installing a new sill—a $1,500 to $4,000 job.
A home speaks through its surfaces. Among the most articulate—and often ignored—is the window sill. As the horizontal ledge protruding from the base of a window frame, the sill bears the brunt of nature’s assault: pounding rain, UV radiation, freezing temperatures, and the mechanical weight of the window above it.
Listen to your home. When the window sill cracks, it is not complaining. It is warning you. Respond accordingly.